Sheathed heating elements having a metal protective sheath and a filling of magnesium oxide powder as packing and insulation for the enclosed resistance element have been subject in the past to undesirable degradation of electrical insulation value during the usual shelf life of the elements.
The usual previous method of manufacturing such heating elements, in the case of copper covered elements, comprised the steps of threading the resistance wire in centered relation in a copper tube constituting the outside cover, followed by pouring powdered magnesium oxide as electrical insulation into the tube to separate the wire from the cover, followed by densification by compression of the magnesium oxide, with subsequent draw-extrusion of the element to a reduced sheath diameter, with consequent further densification of the insulation. The succeeding steps of the method then included heating the element in a furnace to a temperature in the order to 1200 degrees F for a period of 15 minutes followed by cooling in air in order to anneal the copper sheath. The thus annealed sheath could then be formed to close radii for purposes of utilizing the element. Elements produced by this process have suffered from an unacceptable number of rejects, the reject rate being adversely affected by the duration of storage and dampness of the shelf environment. Element rejects were due to a significant reduction in the insulation value of the magnesium oxide caused by the ingress of atmospheric moisture, leading to the failure of an unacceptable percentage of such elements to pass a high voltage ("high pot") test. Corrective action by baking the elements at 500 degrees F. for periods of up to 8 hours was required to remove some of the moisture from the elements before sealing. Even then a considerable amount of moisture remained which led to high current leakages.
One attempted method to counter such deterioration of insulation values has involved spraying of the open sheath ends with a sealer, known as circuit board sealer (DOW CORNING QR-4-3117.TM) for purposes of impeding the entry of moisture therein. This improvement met with only limited success. The treatment of magnesium oxide insulation with silicone sealant has previously been used with steel and aluminum clad elements that did not require annealing before being formed.